1. Field of the Invention
This invention relates to an organic electroluminescent device having excellent light emitting properties.
2. Description of the Related Art
An organic electroluminescent device (which will hereinafter be called "organic EL device") is a light emitting device which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode. Since C. W. Tang et al. of Eastman Kodak Company made a report on a low-voltage-driven organic EL device using a double layered structure (C. W. Tang, S. A. Vanslyke, Applied Physics Letters, 51, 913(1987) and the like), studies on an organic EL device and materials have been briskly carried out.
Tang et al. applied tris(8-hydroxyquinolinol)-aluminum to a light emitting layer and a triphenyliamine derivative to a hole transport layer. The laminate structure is accompanied by such advantages as an improvement in the injection efficiency of holes into a light emitting layer; blocking of electrons injected from a cathode, which increases the efficiency of exciton production from charge recombination; and confinement of the excitons into the light emitting layer. As an organic EL device structure, a double layered device composed of a hole injecting and transport layer and an electron transporting and light emitting layer or a triple layered device composed of a hole injecting and transporting layer, a light emitting layer and an electron injecting and transporting layer are well known. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such a multi-layered device.
As the hole transport material, triphenylamine derivatives such as 4,4', 4"-tris(3-methylphenylphenylamino)triphenylamine known as a starburst molecule and aromatic diamine derivatives such as N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine are well known (ex. Japanese Patent Application Laid-Open Nos. 20771/1996, 40995/1996, 40997/1996, 543397/1996 and 87122/1996).
As the electron transporting material, oxadiazole derivatives and triazole derivatives and the like are well known.
As the light emitting material, known are chelate compounds such as tris(8-quinolinolate)aluminum complex, coumarin derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives, oxadiazole derivatives and the like. Since various color luminescent materials from blue to red region are reported, there is increased expectation for industrialization of a full color organic EL display (ex. Japanese Patent Application Laid-Open Nos. 239655/1996, 138561/1995, 200289/1991 and the like).
In recent days, an organic EL device with high brightness and long life has been disclosed or reported, but it is not always sufficient for manufacturing actual devices yet. Accordingly, there is a strong demand for the development of materials exhibiting high performance.